Ensure all Query statements compile w/ orm, fix test harness

Ensure all Query statements compile w/ orm, fix test harness

Fixed regression where :meth:`_orm.Query.join` would produce no effect if
the query itself as well as the join target were against a
:class:`_schema.Table` object, rather than a mapped class. This was part of
a more systemic issue where the legacy ORM query compiler would not be
correctly used from a :class:`_orm.Query` if the statement produced had not
ORM entities present within it.

Also repair the assert_compile() method, which was using
Query._compile_state() which was bypassing the bug.   A handful
of ORM tests with Query objects and Core-only objects were actually
failing if the default compilation path were used.

Fixes: #6003
Change-Id: I97478b2d06bf6c01fe1f09ee118e1f2ac4c849bc